1. Field of Invention
The present invention is relates to a die casting machine, more particularly relates to a die casting machine using a vacuum die casting method for exhausting a gas inside a die cavity before injecting and filling a molten metal and casting under reduced pressure.
2. Description of the Related Art
One of the reasons for variations in quality of die cast products and the resultant drop in reliability is gas contained in the die cast products. That is, the molten metal injected and filled at a high speed and high pressure forms turbulence inside the injection sleeve and die cavity resulting in entrainment of air or vaporized release agent coated on the die.
To overcome this disadvantage, there is known the technology of casting using a die casting machine using the vacuum die casting method so as to suppress entrainment of gas in the die cast products and thereby reduce variations in quality due to content of gas in the die cast products (for example, see U.S. Pat. No. 2,785,448).
In a die casting machine using the vacuum die casting method, the die cavity has to be able to be made greater in vacuum and the reduced pressure state maintained in order to cast a high strength and quality product. If the die cavity is not made high in vacuum, the cast product may contain gas. As a result, when applying annealing or other heat treatment to the product after casting, the product will easily suffer from warping or deformation and it will be difficult to obtain a sufficient effect by the vacuum die casting method.
Summarizing the disadvantages to be solved by the invention, to reduce the pressure in the cavity, it is necessary to provide a valve in the exhaust path communicating a vacuum pump and the cavity and use this valve to open and close the exhaust path.
From the viewpoint of preventing the drop in vacuum degree in the cavity, the timing of closing the valve is preferably right before injecting and filling the cavity with the molten metal.
On the other hand, if the timing of closing of the valve becomes delayed, the molten metal may enter the valve and the valve may be damaged.
Therefore, it is necessary to optimize the timing of closing the valve so as to obtain a suitable pressure drop and prevent entry of molten metal into the valve.
However, the timing of closing the valve easily fluctuates due to various factors such as the variation in response of valves. Molten metal may therefore enter the valve even if closing the valve at a timing with a certain leeway.